1. Field of the Invention
The present invention relates to a nano data writing and reading apparatus using a cantilever structure and its fabrication method, and particularly, to a nano data writing/reading apparatus using a cantilever structure and its fabrication method configured to improve durability and performance of the cantilever structure and remarkably reduce a fabrication process.
2. Description of the Background Art
In general, an atomic force microscope is a device for measuring the topography of surfaces, using a cantilever structure, and a fine cantilever probe is formed at a front end portion of one side of a cantilever in the atomic force microscope. By using the cantilever probe, the surface topography and an electrical or magnetic characteristic of a sample can be identified with a nanometer scale resolution. The biggest advantage of such an atomic force microscope is its resolution that is high enough to directly measure an atomic structure by using a probe of several nanometers.
Recently, research is actively ongoing on a data writing and reading apparatus having a storage density of more than Tbit/in2 by using the high resolution of the atomic force microscope. A device that writes data upon changing properties of a storage media by using the cantilever probe is called “a data writing apparatus using an SPM(Scanning Probe Microscopy) principle”. As the method of changing the storage media by using the SPM principle, there are a method of mechanically changing the media by using heat, a method of changing a polarity of a ferroelectric such as a piezoelectric driver, a method of inducing a change in resistance by changing a phase with heat or electricity, using a phase transition material, and the like. Besides, a method using a ferromagnetic material or the like is used.
The representative one of the data writing and reading apparatuses using such various SPM principles is “Millipede” developed by IBM. It uses a polymer material (e.g., PMMA) as recording media by applying a principle of an atomic microscope, which is one of the SPM principles. The Millipede is configured such that a plurality of unit cantilevers (64×64) are disposed for the purpose of improving a speed. Such a unit cantilever includes a silicon probe, a heater and a silicon cantilever.
FIGS. 1A and 1B are sectional views of a unit cantilever of the conventional nano data writing and reading apparatus developed by IBM, namely, the Millipede, which shows an operation principle of writing and reading data on and from the media.
First, as shown in FIG. 1A, data is written on or read from the media including a silicon substrate 20 and a polymer part 21 by using a cantilever 10 having a heater 11 formed in contact with the probe 13.
FIG. 1B shows a method for writing data on the media. In the drawing, the writing is performed by forming a hole. The hole is formed such that the polymer part 21 of the media including the silicon substrate 20 and the polymer part 21 is melt by heating a heater 11 formed at an end portion of the cantilever 10.
A method of reading the data written on the media will now be described with reference to FIGS. 1B and 1C. First, when the data is read, the heater 11 is heated and then the reading is performed by using a difference between rates at which the heater 11 is cooled. When the probe 13 is inserted in a hole formed as shown in FIG. 1B, a distance (a) between the heater 11 and the media 20, 21 becomes short, and heat is diffused to the media 20, 21 through the probe, which makes a cooling rate of the heater 11 high. In contrast, when the probe 13 is placed on the surface of the media 20, 21 as shown in FIG. 1C, a distance (b) between the heater 11 and the media 20, 21 becomes long, which makes the cooling rate of the heater 11 low. In such a manner, it can be determined whether or not the hole exists, so that the data can be read.
Such a writing and reading method is called a thermo-mechanical method.
FIGS. 2A and 2B are views which specifically illustrate the method of writing data by using the cantilever employing the thermo-mechanical method. As shown in the drawings, a range of a region within which the data is stored is varied according to a thickness of a polymer of recording media. Namely, as shown in FIG. 2A, if a thick polycarbon 21a is used as media, a region (dbit) for writing becomes large, thereby lowering a writing density. In contrast, as shown in FIG. 2B, if a thin PMMA 21b is used as the media, a region (dbit) for the writing becomes small, thereby increasing the writing density. Accordingly, the PMMA 21b having a high density is commonly used as the media. Also, if the PMMA is used as the media, a time required for the writing is also reduced.
Because it takes a relatively long time to write and read the data by the thermo-mechanical method, a plurality of unit cantilevers are arranged and used as writing and reading head parts in order to improve a writing and reading rate. A method for fabricating a cantilever array head therefor is disclosed in a report issued by IBM in 2003 (Nanotechnology-based Approach to Data storage (RZ 3480, 2003, 08, 25), p. 1907˜1910 of Transducer'03).
FIGS. 3 and 4A˜4F illustrate a core part of the method of fabricating a cantilever array structure released by IBM in 2003. FIG. 3 is a conceptual view of a wafer-level fabrication method, and FIGS. 4A to 4F are sectional views of a unit cantilever, for illustrating sequential processes of the cantilever fabrication method.
FIG. 3 is a conceptual view which illustrates an entire process for forming a data writing and reading apparatus by bonding a signal transfer circuit unit wafer with a cantilever array wafer which are made by two separate processes.
First, fabricating a wafer where a signal transfer circuit unit is formed is constituting an electric circuit by using CMOS. Here, an electronic circuit part processing driving signals and data of a data writing and reading apparatus to be formed later is formed (S10), and a structure for bonding with a cantilever part which is separately formed is formed (S20).
As for the fabrication of a cantilever array wafer, a cantilever structure is formed on a seed wafer to be sacrificed (S01), a glass wafer is bonded with an entire upper surface of the cantilever structure (S02), the cantilever structure is transferred onto the glass wafer by removing the seed wafer (S03), and the glass wafer is flipped over and a structure for the bonding with the wafer having the signal transfer circuit unit is formed (S04).
When the fabrication of the signal transfer circuit unit wafer and the cantilever array wafer is completed, the bonding structures formed at both the wafers, respectively, are made to face each other, and then, the two wafers are bonded by heat and pressure applied thereto (S31). Then, the glass wafer which has supported the cantilever structure is removed (S41). Namely, the cantilever structure is transferred to the signal transfer circuit unit wafer.
In the aforedescribed process, the process of bonding the seed wafer with the glass wafer is required in the cantilever array wafer process, and the process of bonding the signal transfer circuit unit wafer with the cantilever array wafer is also required. Namely, a process difficult to perform such as an alignment process between the structures should be performed twice, which causes problems in that it requires a long time to perform the process, a yield is reduced and a cost is increased.
FIGS. 4A to 4F are views which illustrate the aforedescribed process in the aspect of a unit cantilever in more detail.
In FIGS. 4A to 4C, a cantilever array structure 32 is formed, using a silicon wafer 30 on which an insulation film 31 has been formed (i.e., an SOI wafer (Silicon On Insulator)), and a glass wafer 34 having an coefficient of thermal expansion similar to that of silicon is bonded with the SOI wafer having the cantilever array structure 32 by using a polyimide layer 33 (FIG. 4A). Then, the silicon substrate 30 and the insulation layer 31 are removed from the SOI wafer (FIG. 4B).
A bonding structure 35 including a bonding pad and a bonding bump are formed on the exposed cantilever array structure 32 so as to be bonded with a wafer 40 having a signal transfer circuit unit for controlling the cantilever array structure 32 and transferring a signal. Such a bonding structure may be formed as two layers of polyimide such that a standard polyimide layer is formed and an adhesive polyimide layer is further formed thereon (FIG. 4C), thereby preparing for a bonding process with the signal transfer circuit unit wafer 40 to be performed thereafter.
FIG. 4D is a view which illustrates a structure of the signal transfer circuit unit wafer 40 formed by a separate process. On the signal transfer circuit unit wafer 40, an electric circuit unit for signal transfer is formed, and a bonding structure 41 for electric coupling between the electric circuit unit and the cantilever structure is formed. The bonding structure 41 is a metal stud formed including a part for bonding and a part for electrical connection.
FIGS. 4E and 4F show a process in which the two wafers having passed through aforementioned processes are bonded together. First, the two wafers are aligned such that the bonding structure 41 formed on the signal transfer circuit unit wafer 40 is inserted between the bonding structures 41 formed on a lower portion of the cantilever array structure 32. Then, bonding is performed thereon under proper temperature and pressure (FIG. 4E). Also, the glass wafer 34 and the polyimide layer 33 which have supported the cantilever array structure 32 are removed (FIG. 4F).
However, in the nano data writing and reading apparatus using the conventional cantilever structure and its fabrication method, because the cantilever structure and the probe are formed by etching a part of a silicon device layer of the SOI wafer, variations in a thickness of the initial silicon device layer and in an etching rate occur, which makes it difficult to maintain a constant thickness of the cantilever structure. Also, because the probe formed of silicon is badly abraded from use, it is difficult to maintain reliability.
Also, in the conventional method for fabricating the cantilever structure, the process of bonding the glass wafer with the SOI wafer where the cantilever structure is arranged, and the process of bonding the wafer having the signal transfer circuit unit with the patterned SOI wafer should be performed. Such bonding processes greatly affect an entire yield of a product. Namely, because such a bonding process of a wafer level should be performed twice in the conventional method, an entire process becomes complicated, a yield of a final product is reduced, and a cost is increased.
If an SOI wafer having epi-silicon is used, variations in the thickness of the cantilever can be reduced, but problems of a large increase in cost and abrasion of a probe still remain.